Devices for the injection of implants have been described earlier in the patent literature. As examples of patent publications disclosing various injection devices for implants can be mentioned EP 304700, EP 304107, WO 8806905, U.S. Pat. No. 4,451,254 and GB 2199247. EP 304700 discloses a device the sterility of which has been improved by preventing the plunger from accidentally coming out from the housing. The housing is made of plastic but the cannula and the plunger are made of metal. EP 304107 discloses a device intended for once-only use for injecting implants in which device the housing is made of plastic but the cannula is made of metal. The patent concerns particularly an element connected with the plunger, the function of which element is to prevent the implant from being pushed forwards too early, i.e. during the puncture of the skin. WO 8806905 relates to a complicated device intended for repeated use for subcutaneous implantation in a desired manner of a plurality of sucessive implants situated in said device. The invention concerns particularly equipment for the application of the implants. U.S. Pat. No. 4,451,254 discloses also an implanter for the application of several implants said implanter being intended for repeated use, wherein the implants are fed from a cartridge mounted to the side of the implanter. The publication GB 2199247 describes equipment for the implantation of hormone implants wherein said device, which is intended for once-only use, is completely made of plastic. First an incision is made into the skin with a scalpel after which a trocar attached to the cannula is pushed to a desired depth beneath the skin. The trocar, the opposite end of which is blunt, is withdrawn and again inserted in the cannula with the blunt end forwards touching the implant, wherein the blunt end is used as plunger. The implant injection devices described above exhibit several disadvantages and faults.
A great risk related to devices intended for once-only use is therein that these devices may come into the hands of misusers of drugs and related substances. The device intended for once-only use as described in the European patent publication EP 304107 would be very easy to use after the removal of its contents of original implants. The plunger can freely be withdrawn from the medical container after which the container easily can be loaded with new substances. The injection device for implants as described in the patent publication EP 304700 contains a flexible ring fitted onto the inner surface of the medical container, said ring interacting with an annular groove in the plunger. The ring and annular groove retard the back and forth motion of the plunger in the medical container so that the plunger will not accidentally come out from the medical container. They also prevent the plunger from accidentally moving forwards before the implants are going to be administered. However, if greater pushing or pulling forces are applied to the plunger, the plunger is enabled to move both inwards and outwards and there is nothing preventing the plunger from being completely drawn out. The reuse of the device intended for once-only use as described in GB 2199247 is not prevented by any means.
The sterilization of the devices for once-only use is preferably performed by gas, e.g. ethylene oxide gas, at the stage when all the components including the implants are fitted in the implant container. Sterilization by gamma radiation represents in principle another alternative for sterilization. It suffers, however, from the disadvantage due to the fact that it is not tolerated by all pharmaceutical substances. As a result thereof, the possibilities of application of gas sterilization are much wider. The once-only usage device according to the EP 304107 patent cannot be sterilized by gas after putting the parts together because there are no passages through which the gas would be permitted to penetrete into the inner part of the device.
The manufacturing costs of devices for once-only use must of course be low. It would be desirable to have all components of the device manufactured of plastic. The cannula creates a problem because it must be equipped with a sharp edge to enable the incision of the skin. In all the constructions described in prior art except for the decive disclosed in GB 2199247 the cannula has been made of metal. The device for once-only use according to EP 304107 is otherwise made of plastic but in the manufacturing of the cannula it has been necessary to use metal as manufacturing material.